The sensors will be placed at 3 different locations at a spiral staircase, and should be activated when people are present in it's area.

I got suggested that I could use surveillance cameras as sensors, since they have a built-in trigger for when movement is detected, and they should be more precise and reliable.I also thought about using webcams, but since a computer won't be permanent available in the setup that won't be a solution.

I am going to work on a project, which requires reliable movement detection.It needs to work for approx. 1 year, with multiple movement detectors a day.

No problem. Just put it within range of an electrical outlet.

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The sensors will be placed at 3 different locations at a spiral staircase, and should be activated when people are present in it's area.

And will the sensor know that there is someone near, if it is not active? I think you have that concept backwards. The sensor is always active, monitoring for motion/heat/whatever. It outputs a different value if there is something present than when there is not.

What is it that you're trying to detect movement of? If it's people, do you care about knowing how many/where they are, or just that there is somebody moving? In the latter case, the solution adopted for every burglar alarm and security lamp seems to be a PIR sensor and that suggests it's the simplest and most reliable approach.

I only provide help via the forum - please do not contact me for private consultancy.

And will the sensor know that there is someone near, if it is not active? I think you have that concept backwards. The sensor is always active, monitoring for motion/heat/whatever. It outputs a different value if there is something present than when there is not.

- Yes it would run 24/7, which means they should be detect the presence of someone anytime.

There are PIR sensors readily available with all the necessary control circuits to switch lamps on, with controls to let you select how long the lamps should be kept on for after the movement stops. Commonly they also have an ambient light sensor and can be configured to stay off when the light is above a specified threshold. You can get light fittings with these PIR sensors and control circuits integrated.

Is there anything stopping you from using these commercial off the shelf units?

I only provide help via the forum - please do not contact me for private consultancy.

I saw that you stated: "I have thought about using the MaxSonar sensor (http://www.adafruit.com/blog/2009/06/23/getting-started-with-the-maxbotix-sonar-sensor-quick-start-guide/), but I don't know if it will be precise enough. The sensors will be placed at 3 different locations at a spiral staircase, and should be activated when people are present in it's area."

You may want to consider using one of our ProxSonar sensors. We have them available in either a USB or Pin Out interface depending on the interface you want to use. Both sensors have a pin that can be monitored that gives a digital high/low signal if a person is in the detection zone of the sensor. The interface with the pin out configure is able to operate within 2.5V to 5.5V and the USB only operates from the voltage supplied by the USB port. Our HRLV-MaxSonar-EZ may be a good rangefinding sensor for your application also. There is some multi-sensor operation built into the sensor. We are currently testing these parameters yet at our last trade show, we had 4 sensors (3 ProxSonars and 1 HRLV-MaxSonar) operating at our booth independently without interference in the reported range.

The LV-ProxSonar-EZ ultrasonic proximity sensor can be seen here http://maxbotix.com/Ultrasonic_Sensors/Ultrasonic_Proximity_Sensor.htm.The USB-ProxSonar-EZ ultrasonic USB proximity sensor can be seen here http://maxbotix.com/Ultrasonic_Sensors/USB_Proximity_Sensor.htm.

If you are looking for the range to the person in the stair well you may want to test the HRLV-MaxSonar-EZ. The sensor can be connected to Arduino or other control device that is programmed to ignore distances after a certain amount. The HRLV-MaxSonar-EZ can be seen here http://maxbotix.com/Ultrasonic_Sensors/High_Resolution_Sensors.htm.

Please let me know if you have any questions by emailing me at scott@maxbotix.com. I hope this information help.